The outer layer of skin surrounding the body performs an important protective function as a barrier against infection, and serves as a means of regulating the exchange of heat, fluid and gas between the body and external environment. When skin is removed or damaged by being abraded, burned or lacerated, this protective function is diminished. Areas of damaged skin are conventionally protected by the application of a wound dressing which facilitates wound healing by acting as a skin substitute.
Wounds to skin and the underlying tissues of humans and animals may be caused by external insult such as friction, abrasion, laceration, burning or chemical irritation. Damage to such tissues may also result from internal metabolic or physical dysfunction, including but not limited to bone protrudence, diabetes, circulatory insufficiencies, or inflammatory processes. Normally tissue damage initiates physiological processes of regeneration and repair. In broad terms, this process is referred to as the wound healing process.
The wound healing process usually progresses through distinct stages leading to the eventual closure, and restoration of the natural function of the tissues. Injury to the skin initiates an immediate vascular response characterized by a transient period of is vasoconstriction, followed by a more prolonged period of vasodilation. Blood components infiltrate the wound site, endothelial cells are released, exposing fibrillar collagen, and platelets attach to exposed sites. As platelets become activated, components are released which initiate events of the intrinsic coagulation pathway. At the same time, a complex series of events trigger the inflammatory pathways generating soluble mediators to direct subsequent stages of the healing process.
Normally, the wound healing process is uneventful and may occur regardless of any intervention, even in the case of acute or traumatic wounds. However, where an underlying metabolic condition or perpetual insult such as pressure or infection are contributing factors, the natural wound healing process may be retarded or completely arrested, resulting in a chronic wound. Trends in modern medical practices have shown that the wound healing of both acute and chronic wounds may be significantly improved by clinical intervention using methods and materials that optimize wound conditions to support the physiological processes of the progressive stages of wound healing. Key factors in providing the optimal conditions are the prevention of scab formation, prevention or control of microbial activity, and the maintenance of an optimal level of moisture in the wound bed. It is also helpful to manage wound exudate fluid.
A common problem in the management of both acute and chronic wounds is the maintenance of an optimal level of moisture over the wound bed during heavy exudate drainage. This is usually, but not always, an early stage of healing. Most moist wound dressing technologies such as thin films, hydrocolloid dressings and hydrogels are typically overwhelmed by the accumulated exudate moisture during this heavy drainage phase. Management of moisture during heavy exudate drainage often necessitates the use of gauze or sponge packings that wick away excess moisture from the wound bed, thin film coverings that trap exudate fluid over the wound bed, or calcium alginate dressings that chemically bind exudate moisture due to the hydroscopic properties of the seaweed extract.
Examples of wound dressings that have been developed include collagen dressings, a natural polymer. Soluble collagen has been used as a subcutaneous implant for repairing dermatological defects such as acne scars, glabellar furrows, excision scars and other soft tissue defects. Collagen has also been used in many forms as wound dressings such as collagen sponges. Several inventions have attempted to solve the problem of maintenance of an optimal level of moisture in the wound environment. Collagen is used as the matrix material in Artandi, U.S. Pat. No. 3,157,524 and Berg et al., U.S. Pat. No. 4,320,201. However, most of these dressings are not satisfactory for the various types of full thickness wounds. Collagen films and sponges do not readily conform to varied wound shapes. Furthermore, some collagen wound dressings have poor fluid absorption properties and undesirably enhance the pooling of wound fluids. Generally, most wound dressing materials do not provide for the control or elimination of microbial bioburden in the wounds.
Another example of wound dressings that have been developed for control of moisture levels in wounds are the hydrocolloid dressings. United Kingdom Patent Number 1,471,013 and Catania et al., U.S. Pat. No. 3,969,498 describe hydrocolloid dressings that are plasma soluble, form an artificial eschar with the moist elements at the wound site, and gradually dissolve to release medicaments. These dressings comprise a hydrophilic foam of dextran polymer that can be applied without therapeutic agents or ointments, are non-irritating to the lesion and can be easily removed.
Known hydrocolloid dressings in general, and the Catania et al. dressings in particular, are subject to a number of drawbacks. The major disadvantages of these dressings include the potential to disintegrate in the presence of excess fluid at the wound site, and minimal, virtually negligible, control over water loss from the wound. This latter disadvantage is particularly important, as excess water loss from a wound will cause an increase in heat loss from the body as a whole, potentially leading to hypermetabolism. In addition, hydrocolloid dressings require frequent dressing changes. This is especially true of the Catania et al. dressing due to the dissolution of the dextran polymer at the wound site caused by the fluid loss through the wound in the exudative stage.
Although currently available dressing materials possess features that contribute to the control of heavy exudate drainage, most also possess significant limitations that retard the overall healing process. For example, thin film dressings such as those described in U.S. Pat. No. 3,645,835, maintain excessive moisture over the wound bed, contributing to the overhydration or maceration of surrounding skin. Although sponges and gauze support tissue, they require frequent changing, and cause irritation to the wound bed during body movement and dressing removal. These dressings may be permeable to moisture but not to microorganisms. Although these devices and others administer some control over wound exudate moisture and may additionally provide a barrier to microbial contamination, they do not actively participate in controlling the growth of microorganisms or in the elimination of microbial bioburden from the wound dressing. Calcium alginates turn into a gelatinous mass during interaction with moisture, are difficult to remove completely, and often dehydrate the wound bed due to the hydroscopic nature of the matrix.
Importantly, none of the presently available devices significantly contribute to or support the autolytic debridement phase, which is the natural removal process of necrotic tissue and debris from the wound. Autolytic debridement is a key early stage event that precedes repair phases of healing. When wound conditions are not optimal for supporting autolytic debridement, then clinical procedures such as surgical removal, irrigation, scrubbing, and enzymatic or chemical methods must be used to remove the necrotic tissue and escar that can inhibit wound healing.
Temporary or permanent wound dressings that are designed to enhance wound healing are needed to cover large open wounds on patients with extensive burns, lacerations and skin damage. Furthermore the ability to produce wound dressings in a variety of shapes to accommodate multiple sizes and forms of injuries is important in the manufacture of useful medical products.
In addition, there continues to be a need for a wound dressing that possesses high moisture absorption capacity, a high rate of absorption, as well as a capacity to regulate moisture at the wound bed-dressing interface. Desirably, such a wound dressing device should stimulate the autolytic debridement process, especially during the heavy exudating phase of wound care management.
Another desirable aspect of a wound dressing would be the ability to deliver active agents to the site of injury to accelerate wound healing and in particular to control the growth and damage caused by microbial contaminants of the wound. Active agents for use in wound treatment may be administered to an individual in a variety of ways. For example, active agents may be administered topically, subingually, orally, or by injection (subcutaneous, intramuscular or intravenous). Nevertheless, there are drawbacks to many of these methods, and an inexpensive, reliable, localized and relatively pain-free method of administering an active agent has not been provided in the prior art.
One common method employed for the treatment of wounds is the topical application of a salve or ointment. Yet many times, topical application to a wound can be painful and short-lived. Additionally, in the case of a deeply cavitated wound in particular, an excess of active agent may be required because the agent must diffuse through layers of necrotic tissue and newly forming epidermal tissues. This difficulty in delivering the agent may require the application of an excessive amount of the agent and preclude an accurate determination of the effective amount of active agent to be added.
The oral and sublingual administrations of active agents used in wound treatment also have their drawbacks. Most importantly, the administration site, the mouth, is normally far removed from the actual location of the wound. Ingestion of an active agent at a site distant from the wound may result in the agent having negative system-wide effects and possibly knocking out the normal flora, or normal microbial environment, whose presence benefits an individual. Successful absorption of the agent into the bloodstream also depends on several factors such as the agent's stability in gastrointestinal fluids, the pH of the gastrointestinal tract, solubility of solid agents, intestinal motility, and gastric emptying.
Injection of an active agent, a normally painful method of administration, may have the same negative system-wide effects as that of an oral or sublingual administration if injection is at a site distant from the wound. Yet more importantly, a danger inherent in the injection of an active agent is that rapid removal of the agent is impossible once it is administered. There is also a risk of transmission of infections and the possibility of vascular injury due to the use of needles.
One active agent, silver, has long been recognized for its broad spectrum anti-microbial activity and compatibility with mammalian tissues. Although silver has been used in a large range of medical devices, its incorporation, as a prophylactic anti-infective agent, in primary wound contact products has been restricted due to silver's adverse properties. These properties include a short half-life, the rapid inactivation of silver by protein, and light-mediated discoloration of the product containing silver and any body parts touching the product, such as skin. Recently, manufacturers have tried methods to overcome some of the limitations to broaden the utility of silver in wound care. The currently available silver-containing wound care dressing materials have been unsuccessful in adequately overcoming the problems inherent in using silver.
Medical devices that are implanted or those that are attached to epithelia may create an environment conducive to the multiplication and growth of microorganisms. This microbial growth may lead to complications such as local or systemic infection. Dermal wounds are at particular risk since microbial contaminants are commonly present, and the wound produces optimal nutrients and other environmental conditions for microbial growth. Medical practices have demanded the use of sterile or low bioburden devices and the adoption of procedures and adjuncts such as frequent dressing changes, use of topical antimicrobial compounds, and systemic antibiotics to control growth of microorganisms on and around the device during use.
An alternative approach is the production of devices that possess broad spectrum antimicrobial activity. A variety of approaches have been taken to endow devices with antimicrobial properties including soaking of indwelling catheters and other devices in antibiotics such as penicillin or fluconazole, or in antiseptic solutions such as chlorhexidine or sulfadiazine. Although these approaches render some antimicrobial activity to the devices, they are of limited utility due to toxicity, stability and effectiveness. Such limitations include short half-lives in tissue or on the devices, the agents' spectrums of activity are too narrow for the range of organisms that may be encountered near the device, or the agents may be destructive to tissues at their effective concentrations.
Heavy metals may provide an optimal alternative as antimicrobial agents for rendering medical devices with antimicrobial properties. Heavy metals may exist as salts, complexes with carriers, as base metals or other forms. This versatility contributes to the variety of ways in which the forms can be coupled with the devices. In addition, it is known that heavy metals such as gold, platinum, silver, zinc and copper exert antimicrobial activity at very low concentrations against a broad spectrum of organisms including bacteria, protozoa, fungi and viruses (N. Grier, “Silver and its compounds” In Disinfection, Sterilization, and Preservation, (3rd edition S. S. Block, ed., Lea & Febiger, Philadelphia, Ch. 20, (1983).). Silver is oligodynamic, meaning that it has antimicrobial activity at very low concentrations against a wide range of bacteria, fungi and viruses. Measurements of ionic silver as low as 10−6 to 10−9 M have been shown to be antimicrobial (A. D. Russel and W. B. Hugo, “Antimicrobial Activity and Action of Silver”, Prog. in Med. Chem. 31:351-370, 1994.) Moreover silver is well tolerated by mammalian cells and tissues.
One active heavy metal, in particular silver, has long been recognized for its broad spectrum anti-microbial activity and compatibility with mammalian tissues. Although silver has been used in a large range of medical devices, its incorporation, as a prophylactic anti-infective agent, in primary wound contact products has been restricted due to silver's adverse properties. These properties include a short half-life, the rapid inactivation of silver by protein, and light-mediated discoloration of the product containing silver and any body parts touching the product, such as skin. Recently, manufacturers have tried methods to overcome some of the limitations to broaden the utility of silver in wound care. The currently available silver-containing wound care dressing materials have been unsuccessful in adequately overcoming the problems inherent in using silver.
The mode of action of silver is due to the reactivity of the ionic form with a variety of electron donating functional groups that contain reactive entities such as oxygen, sulfur or nitrogen. Electron donating functional groups in biological systems are many and varied, including groups such as phosphates, hydroxyl, carboxylates, thiol, imidazoles, amines, and indoles. Microbial macromolecules are richly endowed with these functional groups that, when bound by silver ion, may become inactivated and disfunctional resulting in the death of the microorganism. Ionic silver is known to disrupt microbial cell wall, cell membrane, electron transport, metabolic and anabolic enzymes, and nucleic acid function (A. D. Russel, W. B. Hugo, “Antimicrobial activity and action of silver” In Progress in Medicinal Chemistry. Vol. 3, G. P. Ellis & D. K. Luscombe, ed., Elsevier Science B. V., (1994)).
Oligodynamic silver has been incorporated into medical inventions for the purpose of imparting an antimicrobial effect. The use of metallic silver was reported in UK patent application No. 2134791A which describes the vapor deposition of metallic silver or silver/carbon on Sphagnum moss for the purpose of making an antimicrobial surgical dressing. U.S. Pat. No. 5,753,251 describes the production of a wound contact product by sputter coating silver on to substrates such as plastic films to impart antimicrobial activity to the device. A description of a metallized bandaging material, prepared by vapor coating metallic silver onto a fiber fleece was described in U.S. Pat. No. 2,934,066. U.S. Pat. No. 4,483,688 describes the combining of finely ground metallic silver with a binding agent for coating indwelling catheters.
Alternative means of incorporating silver or silver salts into or on devices have also been described. The incorporation of antimicrobial silver into the adhesive of an adhesive coated, moisture impermeable thin film polymer for use for securing medical devices or as a wound dressing was described in U.S. Pat. No. 4,340,043. The use of silver oxide, finely ground into small particles, dispersed in latex batching has been described in U.S. Pat. No. 4,902,503 for use in making indwelling medical devices where antimicrobial activity would increase effectiveness.
Although these devices have provided certain solutions to combining antimicrobial activity with medical devices, these inventions have identified a number of limitations associated with silver and silver salts. The highly reactive nature of silver ions contributes to the relatively short half-life of the antimicrobial effect in the presence of certain functional groups. Moreover its antimicrobial form, ionic silver, is unstable in light and is rapidly converted to a black inactive precipitate by photo-reduction.
Attempts at overcoming the limitations of silver addition included applying silver or silver salts onto dry substrates where little or no ionization of silver could occur or the use of substrates containing few reactive functional groups that would react with ionic silver. However, this is impractical for applications where moisture abounds such as in moist devices such as soft contact lenses, hydrated plastic implants, or in moist wound dressing cover such as a hydrogels, hydrocolloids, or biologics, or in medical devices that contain reactive functional groups such as in a collagen matrix.
To overcome these problems, inventions describing stabilization of silver have been described. U.S. Pat. No. 5,863,548 describes the process of forming a complex between silver and allantoin which in turn is encapsulated in allantoin to form a light stable antimicrobial coating for medical devices. U.S. Pat. No. 5,709,870 describes a process for producing a light and heat stable silver complex with carboxymethyl cellulose for use in coating fibers. Similarly U.S. Pat. No. 5,744,151 describes a process for rendering silver photo-stable and antimicrobial for use as an adjunct to pharmaceuticals by forming an acyclic polyether polymer stabilized by ratios of cation and anions in the process.
The stabilization of the antimicrobial effect of silver in a device that is exposed to light or is in contact with functionally reactive groups may also be accomplished by retarding the release of the silver ion into the environment around the device after application. In other words by using mechanisms that continuously release a small steady supply of ionic silver into the device. An invention described in U.S. Pat. No. 5,470,585 incorporates silver into a form of glass that slowly dissolves in the presence of moisture. The slow dissolution of the matrix thereby releases ionic silver about the device. Sputter coated nanocrystalline silver coatings on devices such as plastics for wound care initially are similarly slowly released from the device during contact with moisture of tissues to liberate ionic silver around the device during use as is described in U.S. Pat. No. 5,753,251.
These inventions have provided some solutions to the problems of stability, and half-life for silver for several silver antimicrobial applications. However they are cumbersome, may contain toxic accessory agents that support function, or are prohibitively expensive for application to commodity medical devices such as wound dressings. Moreover, these approaches are not solutions to the incorporation of antimicrobial silver into devices that contain solvents where ionization of the silver would normally occur in wound dressings such as hydrogels, moist contact lenses, oral prosthetics and other devices containing water. In addition, these inventions make only marginal contribution to the sustained continuous release of ionic silver from devices treated by the processes described. What is needed are compositions and methods for providing antimicrobial activity in medical devices, and particularly for silver incorporation into medical devices such as moisture-containing wound dressings, skin contact devices, such as monitor leads, wound dressings and hydrated plastic implants.